The present invention relates to apparatus for determining the properties of a lubricant and specifically to one which is of relatively simplified construction wherein the film-forming properties of a comparatively small amount of lubricant with roller contact can be determined quickly and without sizeable operating and monitoring equipment. The test apparatus of the present invention is characterized by novel features of construction and arrangement wherein the extraneous forces attributable to the measurement apparatus masking the true properties of a lubricant under evaluation are negligible and thus the evaluation is highly accurate.
There are presently test machines for lubricants which permit examination of the behavior of machine parts moving against each other under high loads.
A suitable lubricant for a given roller bearing is determined by considering the elasto-hydrodynamic theory of lubrication, commonly called the EHD theory. The pressure dependency of viscosity considering the very complicated pressure and temperature phenomenon in the lubrication gap has a decided effect on friction and wear. The magnitude of lubricant characteristics such as viscosity as a function of pressure and temperature, shape of the lubricating gap (lubricating film thickness) and others cannot be easily detected as to their individual effect.
In these known test devices, the sliding or rolling friction of bodies moving relative to one another is examined whereby the bodies are either moved against each other in one direction or actuated to effect an oscillating movement. A known test machine of this type is the so-called two-disc test bench wherein friction for certain operating conditions can be measured quickly and accurately. In this unit, two cylindrical discs contact each other at the periphery and are both driven independently of each other with an infinitely variable adjustment. The frictional force at the contact area produces a parallel displacement of the upper test disc together with its support bearing since the support bearing is hinged with only two leaf springs to the test bench frame. The frictional force acts against a force measuring strip and is measured with strain gauges. The contact pressure between the discs is applied by calibrated springs. The number of disc revolutions is then recorded with a photo cell and indicated on an electronic counter. Simultaneously with the friction measurement, the lubrication condition is constantly checked by measuring the current transfer at the location of contact. The individual current transfers during contact of roughness peaks and their distribution over the periphery are indicated on a cathode ray oscillograph. This apparatus and procedure are set forth in a German publication entitled "Konstruktion", 23rd Edition dated July, 1971 starting at Page 245.
There are also prior known test apparatus wherein the effects of dynamic forces on the bearing surfaces can be examined, such as the effect of vibrations. During a test on lubricants, the operating temperature, the time span for certain damage, the surfaces of elements moved against each other, more commonly known as wear and the co-efficient of friction are measured. After running the tests the appearance of the running marks in the contacting bodies such as rolling elements is evaluated.
In another known device for determining the lubricating properties of a liquid, the lubricant is applied to a rotating plate to form a film and a sensor is brought into contact with the rotating plate to measure the elapsed time to the breakdown of the lubricant as the result of frictional heat. A sensor is arranged at the free end of an elastic arm fastened to a pivotally mounted frame which is moveable in the plane of the plate by reason of its elasticity. Additionally, indicating means are provided for detecting the angular movement of the sensor at the moment of breakdown of the liquid film. This system is illustrated in German Pat. No. 2,034,812.
Even though these prior known apparatus are generally suitable for the purposes intended, they have a number of deficiencies and drawbacks. For example, tedious and time consuming investigations involving large amounts of lubricant are required in order to obtain relatively accurate results or statements concerning the film forming properties of the lubricant. For this reason it is usually necessary to employ several apparatus and this requires a relatively sizeable operating and monitoring effort.
With the foregoing in mind, it is an object of the present invention to provide a simple apparatus wherein the film forming properties of the lubricant with roller contact can be determined quickly and without sizeable operating and monitoring effort with a comparatively small quantity of lubricant. For example, the film forming properties of a lubricant can be determined with test apparatus in accordance with the present invention in a maximum time of 30 seconds and with as little as two grams of lubricant. It is also possible to intentionally "construct" lubricants with the test results gained.
The test apparatus of the present invention incorporates an axial cylindrical roller bearing whereby shape and surface tolerances of the bearing parts are attained which prevent harmful effects on the test results. By supporting an axial cylindrical roller bearing on a hydrostatic cushion with extremely low friction, a measurement of the moment of friction is possible in the simplest way via force pickups to which resistance strain gauges are attached by adhesion.
In order to center the bearing discs of the axial cylindrical roller bearing accurately relative to one another, a radial rolling bearing, in the present instance a ball bearing, is provided as a floating bearing between these two parts. Distance changes of the two bearing discs of the axial cylindrical roller bearing are measured by a capacitive transmitter. This capacitive transmitter measuring means measures film thickness at the center of rotation and is therefore highly tolerant of deviations in the geometry of the test bearing in the connecting parts resulting in highly accurate measuring values.
By reason of the arrangement of the capacitive transmitter in the trunnion-like shoulder surrounded by the bell-shaped top, an "air bubble" is formed so that even when running a test with a large amount of lubricant in the area of the capacitive transmitter and its reference surface the measuring result of the transmitter is not negatively affected by the lubricant. More specifically, by reason of the fact that the capacitive transmitter measuring means is located at the center of rotation, the centripetal force effect tends to keep lubricant out of this region thus providing another insurance against negatively effecting measuring values.